480 



Jlr. R. Kirkpatrick on the 



The Aniphidiscophoran hexadisk is a hexaster with end 

 rays * turned down towards the main rays; in the Hexastero- 

 phora, on the other hand, the end rays of the hexaster are 

 directed upwards away from the main rays. 



X 1400 



Fig. 1. — Dermal surface of Hyalonema tuUpa, F. E. Sch., x 50 (after 

 F. E. Schulze), showing vertical orientation of amphidisks. 



Fig. 2. — Hexadisk of 3Ionorhaphis chuni, F. E. Sch. (after F. E. Schulze). 



Fig. 3. — Monoxyhexaster of Bathydorus uncifer, F. E. Sch., with axial 

 canals only in main rays, and not continued into end rays. X 480. 



Fig. 4. — True microhexactin of Hyalonema divergens, F. E. Sch. X 300. 



Fig. o. — Part of same, examined in glycerine, to show axial canal ex- 

 tending the whole length of ray. X 1400. 



What are the causes which have brought about these funda- 

 mental differences ? The Hexactinellid sponge consists of a 



* It may be objected that the structure at the end of the ray of a 

 hexadisk is a disk with a peripheral ring of teeth, which should not 

 be compared with end rays of a hexaster; but it is here maintained 

 that the teeth round the periphery of the swollen end of the main rays of 

 a hexadisk are essentially of the same nature as the teeth or spines 

 or end rays at the end of the main rays — also often swollen into capitula 

 — of Hex'asterophoran hexasters. The disks of Hexasterophoran disco- 

 hexasters belong to a third order, and their teeth might be regarded as 

 rays of a thii-d order. The disks and rays of amphidisks belong to a 

 second order, and the main rajs of hexasters and amphidisks to a first 

 order. 



